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Passive FIDA Light in BES Signals*

Bill Heidbrink

BES data

G. McKee, D. Smith et al.

NSTX f-FIDA & SSNPA

A. Bortolon

The initial query

E. Fredrickson, M. Okabayashi

*Heidbrink, PPCF 52 (2010) to be submitted

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Thesis

•BES* measures passive FIDA light as well as beam emission

•The combination of beam emission & passive FIDA light complicates interpretation of BES signals during instabilities that cause fast-ion loss

Terminology

“Active” = reactions with injected neutrals

“Passive” = reactions with ambient neutrals

*BES stands for “Beam Emission Spectroscopy” but here “BES” means the diagnostic

FIDA = Fast Ion D-alpha

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Outline

•How FIDA light is detected

•FIDA signals of classical origin

•FIDA signals during instabilities

•Diagnostic Implications

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The NSTX BES diagnostic* measures red-shifted light

• Large Doppler shift from 656.1 nm to avoid carbon lines

• Cold D-alpha eliminated by filter

• Views looks up to align with magnetic field line *Smith, RSI 81 (2010) 10D717.

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|| || ||( / ) ( , , ( / )) ( , , ( / ))Signal d dE d v v W E v v F E v v r r r

W = weight function

F = distribution function

( , )W E pitch

FIDA light from co-going fast ions is transmitted

•Nearly all high-energy fast ions can produce light in the pass band

BES sensitivity to edge FIDA light

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The DIII-D BES diagnostic* measures blue-shifted light from the 150

beams

• Filter band covers FIDA region

• Some cold D-alpha leaks in

• Midplane installation*Gupta, RSI 75 (2004) 3493.

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BES diagnostics on both machines are sensitive to co-going fast ions

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f-FIDA and FIDA imaging use the same principal as BES

Bandpass filter one side of the spectrum

Heidbrink, RSI 81 (2010) 10D727.

or CCD

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Beam emission is usually much larger than FIDA

•Signal levels calculated by FIDASIM* for an XP1014 discharge negligible FIDA contribution to BES signals

*Heidbrink, Comm. Comp. Phys. 8 (2010) submitted

14/ 2 10F B fI I nActive FIDA: Beam Emission =

nf is the fast-ion density (cm-3)

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Passive FIDA light can transiently equal beam emission

140, 0,/ 2 10 ( / )F B edge inj fI I n n n

Passive FIDA: Beam Emission =

•The edge density is an order of magnitude larger than the injected neutral density

•Ordinarily, nf is tiny in the edge

•When fast ions are expelled, the edge fast-ion density is comparable to the core fast-ion density

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Outline

•The BES filter transmits FIDA light

•FIDA signals of classical origin

•FIDA signals during instabilities

•Diagnostic Implications

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Use Beam Blips and Current Ramps to Calibrate Loss Detectors

• Blip all 8 sources

• Ramp Ip to alter loss orbit

• See signal when orbit “connects” deposition point to loss detector

• BES measures beam emission when 150 beams blip onc.f., Zhu, NF 50 (2010)

084024; Pace, RSI 81 (2010) 10D305

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See signal when prompt loss orbit “connects” deposition & loss phase

space• Co-

going losses from 210RT hit BILD

• Current shifts losses toroidally

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BES sees prompt losses from 210RT

• Beam emission order of magnitude brighter

• Seen by all channels

• Occurs for wider range of Ip than for loss detectors

• Low-pass filter rounds signal

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BES sees prompt losses from 210RT

• Co-going losses from 210RT hit edge in BES sightline

• Less sensitive to Ip because many velocities in passband

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BES signals persist after beams turn off

• Beam emission turns off immediately

• Large gyroradii in 3.7 kG plasma fast ions in edge

• Edge fast ions decay in a few ms

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Outline

•The BES filter transmits FIDA light

•Classical fast-ion loss produces detectable BES signals

•FIDA signals during instabilities

•Diagnostic Implications

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Off-axis fishbones in DIII-D: Losses have a definite phase relative to the mode

•BILD saturated on most bursts

•Relatively weak burst

•Like “beacon” measured for classic fishbones

•Phase consistent with Ex Bconvective transport

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BES signal during fishbone combines beam emission and passive FIDA bursts

•BES often saturates

•Correlates temporally with largest bursts on loss diagnostics

•Positive signal grows much faster than negative signal

•If beam emission, implies 150% density fluctuations in core!

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Bursts are observed when 150 beams are off

•No beam emission possible

•All channels see bursts of similar amplitude

•Two peaks per cycle usually observed

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Two bursts per cycle because sightline intersects edge twice

•Phase difference of ~ 700 between bursts

•Approximate toroidal separation of edge crossings

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NSTX BES signal combines BES oscillations and passive FIDA bursts

• TAE avalanche triggers fishbone then n=1 mode

• BES looks like superposition of Mirnov and SSNPA signals

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Elevated BES signal after TAE avalanches

• BES signal oscillates at mode frequency (usual ne fluctuation)

• Bump when fast ions expelled resembles passive f-FIDA signal

• Incoherent losses after a burst common of f.i. signals

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Outline

•The BES filter transmits FIDA light

•Classical fast-ion loss produces detectable BES signals

•Expelled fast ions can produce large BES signals

•Diagnostic Implications

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Negligible impact on BES turbulence measurements

•Broad microturbulence spectrum

• Fast-ion transport usually small few ions reach scrape-off passive FIDA << beam emission

•Non-resonant interaction implies low-frequency response tiny effect on fluctuation spectrum

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BES eigenfunction measurements of fast-ion instabilities compromised

•Large, resonant transport to edge coherent fluctuations in edge FIDA light @ mode frequency

• On DIII-D, use “150-beams off” shots to check for contamination.

•Can’t turn off sources in NSTX without altering the instability

NSTX needs a few equivalent toroidally-displaced views!

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Edge fast-ion light affords diagnostic opportunities

•Use different filters to measure energy of lost particles

•Use FIDA imaging* to measure poloidal distribution of losses

•View prompt-loss signal (calibrated fast-ion density) edge neutral density

•ITER alpha-loss diagnostic?+ (estimated signal is large)

*Van Zeeland, PPCF 51 (2009) 055001

+Boivin

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Thesis

•BES measures passive FIDA light as well as beam emission

•The combination of beam emission & passive FIDA light complicates interpretation of BES signals during instabilities that cause fast-ion loss